It is actually a violation of Federal Law for them to withhold your prescription from you.
See here for cite to relevenat FTC rregulation
The counterperson was lying to you, whether it was intentional on her part, or just what the eye doctor told her to say to put people off, is a matter for speculation.
I’m in Tennessee, and I’ve bought prescription eyeglasses on-line several times. The site just asks you to fill in the numbers off the prescription form. I’ve had no trouble at all, and no request to see tthe actual prescription.
Also, I don’t know if this is state law or what, but when I got my last eye exam, the optometrist (is that right?) made a point of giving me my prescription to take with me. I don’t recall her exact words, but she gave me the impression that it was my legal right to have that; I had the feeling she was going out of her way to comply with some regulation.
By the way, GlassyEyes.com is a big proponent of getting glasses on-line and often links to special deals and discounts. The guy is not affiliated with any retailer and the site has reviews of the various outlets and such. If you need new glasses, I’d start there.
She wasn’t withholding the prescription. The doctor wasn’t in the office to write the prescription. I’m sure had the OP returned when the the doctor was in the office, it would have been possible to get a prescription. Or, the counter person could have been asked to have the doctor write the prescription and put it in the file to be picked up later, or have it mailed.
The counter person can’t write the prescription any more than the receptionist at the MD can.
A glasses prescription is not a one-use item like a pharmacy prescription. They should (in fact must) have it on file. I have on many occasions gotten it written down on a little card by the receptionist. The doctor’s signature is not required.
I did return later and they gave me a small laminated card with my prescription on the back; it was signed by the doctor and is supposedly good for one year.
After visiting another eyeglass store, it seems that in order for them to make you a pair of prescription glasses, they must have either your old glasses so that they can read the prescription from them (using a special machine), or a written prescription with an eye doctor’s signature. She said they WILL NOT make glasses for you just based on you telling them what prescription you want.
So I guess you DO need a prescription for prescription glasses. 
I guess maybe that depends on where you are.
They’re not one-time use, but, at least in Ohio, I get them on a form signed by the doctor. I can carry that form around and use it all I want, but it has to be signed by the doctor, and it expires after 2 years. Contact prescriptions expire after 1 year.
I don’t know if the law works differently for on-line ordering, but you definitely do not need a written prescription to order from, for example, 39 Dollar Glasses or any of the other sites listed on Glassy Eyes. You just enter the numbers into their Web-form and they send you glasses. I’ve done it myself.
(Caveat: It was a little over a year ago that I did it, so maybe things have changed. But I went to 39 Dollar Glasses just now and clicked through as though I were going to order a pair, and it just pulled up a Web-form to fill in with the prescription numbers.)
Walmart (at least around here) is already selling basic glasses for $38, so that business plan wouldn’t work so well. 
Yeah, I was quite surprised to find that I didn’t have to prove I had a prescription for my glasses, just had to fill in the curvature numbers. Nice.
I’m in Toronto, Canada and I just tried to purchase glasses at one of the places inside Wal-Mart. So I would not lose my prescription I had a photo of it on my phone. They would not accept it. Since I know they just look at it and enter some numbers in the computer and I’m paying for them myself, I cannot understand this at all.